Twenty-one crewmen were killed in the worst single attack on merchant shipping of the Persian Gulf war, shipping officials said today. Iraqi jets pumped Exocet missiles into the 218,467-ton Norwegian-operated tanker Susangird, owned by the National Iranian Tanker Co., as it sailed fully loaded from the Kharg Island terminal last week.

Iraq said Wednesday that its warplanes raided three tankers off Iran in the Persian Gulf and industrial targets on the mainland. Iran threatened to answer any Western economic embargo by sealing off the Persian Gulf. Iran also said that three of its warplanes approached American warships south of the gulf in defiance of U.S. Navy warnings. Meantime, the 13th convoy of Navy vessels and reflagged tankers arrived safely in Kuwait, which has been hit by three Iranian missiles this month.

A convoy of reflagged Kuwaiti tankers and American warships steamed south Wednesday, past an Iranian oil platform destroyed Monday by U.S. Navy guns. The escorted convoy is expected to complete the 550-mile trip out of the gulf today. It is the 12th such operation in the three months since U.S. warships began escorting Kuwaiti-owned tankers, registered in the United States and flying American flags, to protect them from Iranian attack.

The Silkworm missile is a Chinese copy of a Soviet surface-to-surface missile known as the SS-N-2, or Styx, primarily used as a shipborne offensive weapon or as part of a coastal defense system. The land-based version that Iran has is easily transportable aboard trucks and could be hard to find in a retaliatory raid, military analysts said Friday.

The United States deployed unprecedented sea and air safeguards Sunday as a new convoy of Navy-escorted tankers steamed through the Persian Gulf. The precautions were apparently sparked by concerns that Iran may launch a retaliatory strike at U.S.-flagged shipping. In the Iran-Iraq War, Iran sent a huge missile slamming into a residential area of Baghdad, killing a number of civilians, while Iraqi warplanes attacked an Iranian oil tanker, killing two crewmen. Concern over the safety of U.S.

The United States and several Western European countries have devised a plan to split up minesweeping duties in the Persian Gulf, sources say. According to two separate diplomatic sources, who spoke on condition of anonymity, the region has been tentatively carved up as follows for minesweeping: -- The United States takes the northern half of the gulf, from Bahrain to Kuwait, an area where U.S. sources say a significant number of mines have been laid.

Iraq and Iran attacked each other with air raids and artillery fire Saturday, one day before a Sept. 20 deadline set by Arab states for Iran to accept a U.N. cease-fire resolution or risk further isolation from its Arab neighbors. Iraq, meanwhile, rejected a plea from Japan to end strikes against Persian Gulf shipping and attacked a Cypriot-flag tanker bound for Iran's major oil terminal at Kharg Island, Japanese and gulf shipping sources said.

U.S. warships escorted another convoy of Kuwaiti tankers into the Persian Gulf on Wednesday, steaming past Iranian missile sites and patrol ships without incident as they moved through the Strait of Hormuz and headed up the gulf toward Kuwait.

U.S. warships, relying on deception and speed, today steered the third convoy of reflagged Kuwaiti tankers through the narrow Strait of Hormuz and into the Persian Gulf. The convoy set sail before dawn in a move that caught shipping sources by surprise. Washington had given no indication when the convoy might begin its journey. Sources at Lamnalco, the Dubai-based agent for the Kuwaiti ships, said the American flags for the convoy were still in its office. "Even the flags are still with us.

A new U.S.-escorted convoy of tankers has sailed through the Strait of Hormuz into the Persian Gulf and is heading for Kuwait, witnesses said today. Reporters in helicopters said that the convoy was composed of three tankers and three U.S. warships. Shipping agents said it was maintaining radio silence and the identities of the tankers was not yet known. It was the third inward-bound convoy of Kuwaiti tankers flying the U.S. flag.